Processes for polymerizing acrylonitrile with starch are well known in the art. For example, such processes are shown in U.S. Pat. Nos. 2,922,768; 3,201,336; 3,661,815; 3,669,915; 3,935,099; 3,985,616; 3,997,484; 4,005,040; 4,045,387; and 4,069,177.
Most of these processes make use of amylaceous (i.e., starch-containing) materials such as pearl starch to produce a graft copolymer which forms a highly water absorbent polymeric composition. These starch-containing polymers can absorb water in amounts of 1,000 times or more by weight of water per weight of the polymeric composition. These products are highly useful in many applications including such uses as incorporation in disposable diapers, surgical pads and sheets, paper towels, disposable paper pads and the like.
The above-mentioned U.S. Pat. No. 4,045,387 discloses a process for producing a highly absorbent polymeric composition which is derived from flour such as corn or wheat flour. While the process disclosed therein is purported to produce a product which will absorb from 1,800 to 3,000 times its weight of deionized water, it has been found that when corn flour is used in the process of this patent to produce a polymeric composition, said composition forms a soft gel. That is, the polymeric composition becomes a highly fluid-like gel material as it absorbs the water.
Polymeric compositions formed from pearl starch, however, using some of the known processes form what is known as a hard gel i.e., a gel which assumes a three-dimensional form and exhibits some rigidity. The formation of the hard gel has a number of advantages in certain applications, such as, for example in seed coating and in disposable diapers as well as in other areas. However, it has been found impossible to produce the hard gel from corn flour utilizing the process of the aforesaid U.S. Pat. No. 4,045,387.
Such a process would be advantageous since corn flour is relatively inexpensive in comparison to pearl starch and is readily available without extensive processing of the corn.